msm: mdss: clean-up terminology used for different bus bw votes

Current driver votes for bandwidth on two different kinds of buses,
data (AXI) and register (AHB). However terminology used to track these
voting is not consistent and easily lead to misunderstanding. Clean-up
some of these terminology to make driver code more readable.

Change-Id: I54d636125786876e8326d6c0279b5a76a1591ae9
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>